1. Field of the Invention
The present invention relates to voltage-controlled oscillators, and more particularly, to voltage-controlled oscillators having an added function for switching the oscillating frequency. The voltage-controlled oscillators find exemplary use in high-frequency circuits, such as dual-mode portable telephones.
2. Description of the Related Art
FIG. 2 is a circuit diagram of a conventional voltage-controlled oscillator. The voltage-controlled oscillator 1 is a Colpitts-type oscillating circuit including a transistor Q1. The collector of the transistor Q1 is connected to a power terminal 2 through a stripline L1, which acts as a choke. A capacitor C1 is connected between the collector and the emitter of the transistor Q1, while a capacitor C2 is connected between the base and the emitter of the transistor Q1. An inductive reactance is formed of a stripline L2 connected between the base of the transistor Q1 and the ground, capacitors C3 and C4 which are used for DC blocking, and a variable-capacitance diode VD1. Bypass capacitors C5 and C6 are connected between the collector of the transistor Q1 and the ground. A resistor R1 is connected between the emitter of the transistor Q1 and the ground, while resistors R2 and R3 are provided for dividing a power voltage Vb at the power terminal 2 and for supplying a base voltage to the transistor Q1.
The voltage-controlled oscillator 1 oscillates only when Z1+Z2+Z3=0. In this expression, Z1 indicates the inductive reactance between the collector and the base of the transistor Q1, Z2 indicates the reactance between the base and the emitter of the transistor Q1, and Z3 indicates the reactance between the collector and the emitter of the transistor Q1. Note that this description of the circuit takes account for the "effective" operational interrelationship of components. For instance, Z1 is shown as located between the base of the transistor Q1 and the terminal 3. However, since the transistor Q1 is collector grounded by C6 in the oscillating frequency, Z1 may be regarded as located between the collector and the base of the transistor Q1.
A control voltage Vc is input to an input terminal 3. As this control voltage Vc changes , the capacitance of the variable-capacitance diode VD1 changes accordingly. Since the inductive reactance Z1 changes, the oscillating frequency f.sub.osc of the voltage-controlled oscillator 1 changes. Therefore, the oscillating frequency f.sub.osc of the voltage-controlled oscillator 1 can be changed by changing the control voltage Vc.
Since the oscillation condition is strictly determined by the relationship Z1+Z2+Z3=0, the range in which the oscillating frequency f.sub.osc changes is somewhat restricted. In other words, the oscillating frequency cannot be extended by a large amount by only changing the inductive reactance Z1 between the collector and the base of the transistor Q1.